Single-stage AC-DC power conversion is a low cost and thus popular power supply topology. In a single-stage AC-DC power converter, the AC line voltage is rectified to produce a rectified input voltage that cycles from approximately zero volts to the peak line voltage (e.g., 1.414*120 V in the US) at twice the AC frequency. Single-stage AC-DC switching power converters include a power switch that cycles multiple times during each cycle of the rectified input voltage. Each time the power switch cycles, a pulse of power is delivered to the load. During the bulk of each rectified input voltage cycle, the rectified input voltage level is relatively high such that a significant amount of power is delivered to the load with every cycle of the power switch. But during the “dead” period of each rectified input voltage cycle in which the rectified input voltage weakens and drops to 0 V, the power delivery with each cycle of the power switch is relatively weak.
This weak power delivery during the dead period between the rectified input voltage peaks complicates the output voltage regulation in indirect control topologies that use a sense voltage from an auxiliary winding such as practiced in single-stage flyback power converters using primary-only feedback. Flyback converters are commonly used as the switching power converter in a single-stage AC-DC architecture. But the isolation between the output voltage on the secondary side of the transformer and the primary side of the transformer in a flyback converter complicates its regulation. The output voltage may be sensed using optoisolators but that raises costs and control complication. In contrast, primary-only feedback control techniques determine the output voltage by sampling the reflected voltage on an auxiliary winding at the transformer reset time. When the rectified input voltage is relatively high, the pulse of energy delivered to the load at each cycle of the power switch is relatively robust such that there is a linear relationship between the reflected voltage and the output voltage. But this linear relationship becomes muddied for the “runt” pulses delivered to the load during the dead period between consecutive rectified input voltage peaks. The primary-only feedback loop thus operates with erroneous output voltage information during the dead periods due to the breakdown in the linear relationship between the reflected voltage and the output voltage for the runt pulses.
This degradation for primary-only feedback control systems is problematic in that a single-stage AC-DC switching power converter may be designed for world-wide use to lower costs and take advantage of mass production efficiencies. But the power line cycling varies across the world depending upon the vagaries of a particular country's electrical power providers. For example, the United States operates with a 60 Hz 120 V (RMS) AC main whereas many other countries such as in Europe operate with a 50 Hz 230 V (RMS) AC main. The control loop in a single-stage AC-DC switching power converter designed for world-wide usage must accommodate these diverse inputs while still keeping their output power to the load within the desired regulation. But such regulation is weakened due to the dead periods of the rectified input voltage cycles for feedback systems that use a sense voltage from an auxiliary winding instead of sampling the output voltage directly such as through optoisolators.
Accordingly, there is a need in the art for improved primary-only regulation of single-stage AC-DC switching power converters.